1. Field of the Invention
The present invention relates to a plasma chemical vapor deposition (CVD) process which is used for example to form a protection film of a magnetic recording medium of thin film type or a superconductor film and an amorphous silicon film for a solar cell.
2. Description of the Prior Art
A thin film is produced by a plasma CVD film-deposition process which has been used in many fields.
In the fields of semiconductor and liquid crystal, such a film has been developed as a protection film. For example, a SiO.sub.2 film is used as an insulator film of an LSI. On the other hand, in the field of magnetic recording medium of thin film type, a diamond-like carbon film or a carbon film is used to improve the reliability of a magnetic disk or a deposition tape. Further, in the field of solar cells an amorphous silicon film is used practically for a solar cell. The amorphous silicon film is manufactured with a plasma CVD process.
However, from the aspect of industrial application, it is a problem that the electrode used in the plasma CVD process has to be cleaned often because it becomes dirty after long term plasma discharge. It is a more serious problem that a substance dropped from the dirty electrode makes the plasma discharge unstable and that it is included into the growing film.
For example, in the field of magnetic recording medium, manufacturing methods have been developed to balance the productivity and the reliability of a diamond-like carbon film, as disclosed in Japanese patent application Nos. 321698/1989 and 286197/1990 which describe means to solve the anomalous discharge phenomenon accompanied with a large excess current and the improvement of productivity.
However, it has been recognized now that a brown to black substance adheres to the electrode in the discharge tube and to the inner wall of the discharge tube after long time discharge. The substance peels off to affect the plasma discharge badly and the resultant magnetic tape has small traces of discharge thereon. Especially, this phenomenon is remarkable when a hydrocarbon gas having a large number of carbon atoms and having double bonds is used. Thus, though the film-deposition rate increases to 20 nm/s and the reliability of magnetic tape becomes sufficient, the stability of the production remains as a problem.
Further, in a plasma CVD process with use of opposite electrodes of the capacitor coupling type, a diamond-like carbon film and an amorphous film as a protection film of magnetic disk are manufactured. In this process, the electrode in the side where no substrate is put becomes dirty after a long discharge time, the deposit to the electrode penetrates into the film. This is a cause of the scattering of the film quality of diamond-like carbon film and the conversion efficiency of solar cell.